This invention relates to an improved control device for an elevator driven by an a.c. motor.
According to a known device of this kind, an elevator car is driven by an induction motor to which a current is supplied from an a.c. source of variable voltage and frequency to effect speed control of the motor. This known device is shown in FIG. 1.
In FIG. 1, the numeral 1 designates a three-phase full wave rectifier device consisting of thyristors 1a to 1f connected in turn to an alternating current source R, S, T. The numeral 2 designates a capacitor for smoothing the output voltage of the rectifier device 1. The numeral 3 designates an inverter connected to the direct current side of the rectifier device and made up of diodes 3a to 3f and transistors 3A to 3F. The inverter is designed to convert the direct current into the alternating current by controlling the base electrodes of the transistors 3A to 3F and to render the voltage and frequency of the resulting alternating current variable. The numeral 4 designates a three-phase induction motor driven by the inverter 3. The numeral 5 designates a brake wheel coupled to the motor 4. The numeral 6 designates a brake shoe mounted opposite to the outer periphery of the brake wheel 5 for braking the brake wheel 5 under the force of a spring, not shown. The numeral 7 denotes a brake coil adapted when energized to disengage the brake shoe 6 from the brake wheel 5 against the force of the spring. The numeral 8 designates a driving sheave of a winch driven by the induction motor. The numeral 9 designates a main guy or cable wound about the sheave 8. The numeral 10 designates a car connected to the main cable 9. The numeral 11 designates a counterweight. The numerals 12a to 12c designate contacts of a magnetic contactor connected between the source R, S, T and the rectifier device 1 and designed to be closed and opened when the car 10 travels and comes to a stop, respectively. The numerals 13a to 13c designate contacts of a magnetic contactor connected between the contacts 13a to 13c and adapted to be opened and closed following opening and closure of the contacts 12a to 12c, respectively.
In operation, the brake wheel 5 is pressured by the brake shoe 6 under the force of the abovementioned spring. When the start commands are issued to the car 10, the contacts 12a to 12c are closed, so that the rectifier 1 delivers a d.c. output. When the capacitor has been charged to a predetermined potential, control elements of respective inverter arms, not shown, are sequentially rendered operative in accordance with the desired car direction so that a.c. output signals of variable voltage and frequency are produced in accordance with the phase order corresponding to the prevailing car direction. Thereafter, the contacts 13a to 13c are closed, and the aforesaid output signal is supplied to the motor 4. The brake coil 7 is energized at the same time so that the brake shoe 6 is disengaged from the brake wheel 5. In this manner, the motor 4 is started in the direction determined by the phase order of the input signals. Thus the car 10 starts its travel. The rpm of the motor 4 and hence the speed of the car 10 are controlled by frequency control function of the inverter 3. As the car 10 approaches the floor of destination, the car starts to be slowed down. The contacts 12a to 12c are opened shortly before the car gets to the floor of destination, so that the motor 4 is disconnected from the source. The brake coil 7 is deenergized at the same time and the brake shoe 6 is pressured to the brake wheel 5 under the force of the aforementioned spring for braking the brake wheel 5. At this time, only the control elements of certain predetermined arms of the inverter 3 are rendered operative by the opening of the contacts 12a to 12c so that the charge of the capacitor 2 flows to the motor 4 for applying a d.c. braking torque to the motor 4. In this manner, the car 10 is stopped positively at the floor of destination.
However, when the thyristors are turned on with closure of the contacts 12a to 12c, a large charging current with quick build-up characteristics flows through the capacitor 2, thus occasionally damaging the thyristors 1a to 1f. Moreover, the service life of the capacitor 2 may be shortened where the start and stop operations are repeated frequently, as in the case of an elevator. Reactors or other current limiting devices may be connected to the direct current side of the rectifier device 1 for protecting the thyristors 1a to 1f and the capacitor 2 from such adverse effects. However, such control device tends to be costly.